Down from the mountains and out of the woods for half a day so the ladyfolk can
get hot baths and the menfolk can buy more beer and ammo. With half a week of
camping still to go, we've already drunk and shot more than we thought we
would, so we're obviously having a good time! Nothing killed, nothing intended
to be, but lots of mangled cans and paper targets with holes. We're all half
deaf and at least that hungover.
I thought I'd see if my e-mail was still screwed up, and it seems to be doing a
little better today. Saw this thread and thought I'd put in my two cents'
worth, having had a bit of experience in this area. I do a fair amount of
macro work with old 6x9cm Graphics. I've made measuring sticks from pieces of
mini venitian blinds marked in 1/4 stops for the half dozen lenses I regularly
use. Going strictly from memory, which, like the old grey mare, ain't what it
used to be, the formula is this:
Marked f-stop x lens-to-film distance
Effective f-stop = ___________________________________________
Marked focal length of lens
The lens-to-film distance is easy to figure out with the simple, uncompromised
lenses a "real" camera uses; I just measure from the approximate position of
the diaphragm, which is easily determined, to the film plane. With lenses like
the 90/2 Zuiko and its ilk, your guess is as good as mine about where to
measure from. With the OM gear, I usually just depend on the camera to do the
figuring and let the flat side drag.
Walt, going back to the boonies
--
"Anything more than 500 yards from
the car just isn't photogenic." --
Edward Weston
-------------- Original message ----------------------
From: jking@xxxxxxxxxxxxxx
>
> Thanks for the email Bart,
> I will have a go with the equations.
> Thanks James
>
> >
> > These formulas are the basic ingredients:
> > The direct connection between focal length (f), distance from the object
> > to the optical centre (o) and image
> > distance from the optical centre (i) is given in:
> > 1/f = 1/i + 1/o
> > Normally, the distance scale (d) on a lens is measured from the object to
> > the film plane:
> > d = i+o
> > Now we should know that the magnification ratio (N) can be defined as:
> > N = i/o
> >
> > Some solid handwork shows how we can express o as the result from f and d:
> > o = 1/2 * f * (d/f - SQRT(SQR(d/f) - 4d/f))
> > We also learnt that i = d - o , so:
> > i = d - ( 1/2 * f * (d/f - SQRT(SQR(d/f) - 4d/f)) )
> >
> > Now, the idea was to express N as a finction of f and d:
> > N = i / o =
> > N = (d - ( 1/2 * f * (d/f - SQRT(SQR(d/f) - 4d/f)) )) / (1/2 * f * (d/f -
> > SQRT(SQR(d/f) - 4d/f)) )
> >
> > Nope, I haven't checked this. Try it, a 90mm (f) lens should do 1:1 (N) at
> > a working distance (d) of 360 mm.
> > You do the math :)
> >
> > One additional nastiness is that many lenses (esp. zoom lenses) use
> > floting alements while focusing. Efectively,
> > this means that the focal length of the lens depends on the distance you
> > focus on. This effect is not present when
> > you use a bellow of course.
> >
> > Or you could put a ruler in your composition and see how many mm's on the
> > subject side fit into the 36 mm's that
> > are resembeled by your focusing screen (allright, multiplied by 98% or
> > whatissit).
> tried that! its a pain for wide angles!
>
> >
> > Or follow Chuck's advice, and forget about the math and figure it out in
> > an emperical way. :)
> trying that too!
>
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